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More evidence for water reservoir at EnceladusBY DR EMILY BALDWIN
ASTRONOMY NOW

Posted: 27 November, 2008

Scientists have found more evidence to suggest that the geyser like plumes spewing out from Saturn’s icy moon Enceladus may be sourced from a warm liquid ocean buried deep within the moon.

Using data collected by the Cassini spacecraft's Ultraviolet Imaging Spectrograph, scientists from the Jet Propulsion Lab, the University of Colorado and the University of Central Florida see evidence for vents channelling water vapour from the liquid reservoir to the surface at supersonic speeds, supporting a mathematical model proposed last year.

Are Enceladus' jets sourced by a warm liquid water ocean buried deep within the icy moon? The debate continues. Image: NASA/JPL/Space Science Institute.

"There are only three places in the Solar System we know or suspect to have liquid water near the surface," says Professor Joshua Colwell of the University of Central Florida. "Earth, Jupiter's moon Europa and now Saturn's Enceladus. Water is a basic ingredient for life, and there are certainly implications there. If we find that the tidal heating that we believe causes these geysers is a common planetary systems phenomenon, then it gets really interesting."

On Earth, liquid water exists four kilometres below ice at Lake Vostok and in some 140 other shallower lakes in Antarctica, so the possibility of similar reservoirs on other planetary bodies is not out of the question. Indeed, many groups of scientists working on Cassini data suspect that a liquid ocean is a strong possibility for Europa.

One recent theory proposes that the jets could be violent bursts of volatile ices that suddenly become exposed to space when Saturn’s tidal forces open and close the tiger stripes at Enceladus’ south pole. The new results, however, cast doubt on this idea. Instead the team found more water vapour coming from the vents at times when the theory predicted there should have been less. That is, at times when Enceladus is further away from Saturn, the vents would compress, reducing or shutting off the jets completely.

"Our observations do not agree with the predicted timing of the
faults opening and closing due to tidal tension and compression," says Candice Hansen, the lead author on the project. "We don't rule it out entirely, but we also definitely do not substantiate this hypothesis."

By observing the flickering light of a star as the geysers impaired a direct view of it on two occasions in 2005 and 2007, the team made measurements of the water vapour content and density of the jets. Theory predicted that more water vapour would be recorded in 2005 when the vents were open than in 2007 when they were closed. But Hansen and colleagues found that the 2007 plume was twice as dense as in 2005, the direct opposite of the original prediction. Their results are presented in the 27 November issue of the journal Nature.

Hansen’s work supports an earlier idea that the vents are like nozzles, focussing the water vapour from depth to the surface at supersonic speeds. They propose that ice grains would condense from the vapour and stream through the cracks in the ice crust before heading into space. The team conclude that only high temperatures close to the melting point of water ice could account for the high speed of the water vapour jets.

Whether there is liquid water present still remains uncertain, but if conclusive evidence arises there would be strong implications for Enceladus as a potential environment to support life. Enceladus will remain a high priority target throughout Cassini’s extended Equinox Mission, which will continue until September 2010.

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